This application is based on Application No. 11-128789 filed in Japan, the entire contents of which is hereby incorporated by reference.
The present invention pertains to a recording method and recording device using a heat reversible recording medium, and more particularly to a recording method and recording device using a heat reversible recording medium including a cholesteric liquid crystal compound.
Conventionally, heat reversible liquid crystal compounds that exhibit a solid phase at a room temperature and selectively reflect light of specific wavelengths when they are heated until a cholesteric liquid crystal phase is achieved are known in the art (see e.g., Advanced Materials 1997, 9, No. 14, pp. 1102-1104). Research by the inventors of this invention, has revealed, however, that with these cholesteric liquid crystal compounds, the temperature range and the selective reflection wavelength range that exhibit a cholesteric liquid crystal phase when the temperature is rising are different from those when the temperature is falling.
FIG. 8 shows a typical example which reveals aspects of the inventive concept. The temperature range and the reproducible selective reflection wavelength range are different in the situations when the temperature of the liquid crystal compound rises, that is where it is heated from a solid phase to a cholesteric liquid crystal phase and then to an isotropic phase, in comparison to when the temperature falls, that is where the liquid crystal compound in an isotropic phase changes to a cholesteric liquid crystal phase and then to a solid phase. During the inventor""s research, it was found that with these compounds, the temperature range and reproducible selective reflection wavelength range that exhibit a liquid crystal phase are narrower in general when the temperature rises than when it falls, and in particular, selective reflection of long wavelength light is more difficult to achieve when the temperature of the compound rises.
Where images are written using a heat reversible recording medium comprising one of these cholesteric liquid crystal compounds, writing may be performed using a process in which the temperature is increased to the level at which the compound exhibits selective reflection and is then rapidly cooled. However, this process entails the problem that the selective reflection wavelength range is narrow and limited to the short wavelength side.
On the other hand, where image writing is performed using the temperature reduction process, it is necessary to first raise the temperature of the heat reversible recording medium to the point at which the liquid crystal exhibits an isotropic phase, and then gradually cool the medium down to the level at which the liquid crystal exhibits selective reflection, followed by rapid cooling. However, such a gradual cooling process requires a substantial amount of time for writing, which is not desirable.
An object of the present invention, therefore, is to provide a new and useful recording method and recording device using a heat reversible recording medium including a cholesteric liquid crystal compound, which enables recording using any desired reflection colors. Another object of the present invention is to provide a recording method and recording device using a heat reversible recording medium, which offers a wide selective reflection wavelength range and which can perform image writing at high speed based on simple temperature control.
In order to attain this and other objects, the recording method pertaining to the present invention consists of a recording method using a heat reversible recording medium including a cholesteric liquid crystal compound, wherein the recording method includes a first process in which all or part of the cholesteric liquid crystal compound of the recording medium is fixed in a state in which it reflects light having a first wavelength, and a second process in which part of the cholesteric liquid crystal compound of the recording medium fixed in the first wavelength light reflection state becomes fixed in a state in which it reflects light having a second wavelength that is shorter than the first wavelength.
Additionally, in order to attain this and other objects, the recording method pertaining to the present invention consists of a recording method using a heat reversible recording medium including a cholesteric liquid crystal compound, wherein the recording method includes a first process in which all or part of the cholesteric liquid crystal compound of the recording medium is fixed in a state in which it reflects light having a first wavelength, and a second process in which a different part of the cholesteric liquid crystal compound becomes fixed in a state in which it reflects light having a second wavelength that is shorter than the first wavelength.
Using the recording method pertaining to the present invention, because all or part of the cholesteric liquid crystal compound becomes fixed in a state in which it reflects light having a desired first wavelength in advance, by performing writing so that the liquid crystal will be fixed in a state in which it reflects light having a desired second wavelength that is shorter than the first wavelength, recording using light having the first wavelength and light having the second wavelength may be completed and recording using desired multiple reflection colors may be made.
In the first process, after the cholesteric liquid crystal compound is heated until all or part of it exhibits an isotropic phase, it is cooled to the temperature where it exhibits a cholesteric liquid crystal phase in which it selectively reflects light having a first wavelength. The cholesteric liquid crystal compound is then rapidly cooled such that it becomes fixed in the state in which it reflects light having the first wavelength. Through this process, recording by means of reflection colors that cannot be obtained using the recording method utilizing heating and rapid cooling may be performed.
In the second process, after part of the cholesteric liquid crystal compound of the recording medium is fixed in the first wavelength light reflection state a part thereof is heated until it exhibits a cholesteric liquid crystal phase in which it selectively reflects light having a desired second wavelength that is shorter than the first wavelength, it is rapidly cooled such that it becomes fixed in the state where it reflects light having the second wavelength. Using this method, gradual cooling from an isotropic phase is not needed, and therefore, the temperature control for image writing is made simpler, enabling fast recording.
Alternatively, in the second process, after part of the cholesteric liquid crystal compound of the recording medium is fixed in the first wavelength light reflection state, a different part of the cholesteric is heated until it exhibits a cholesteric liquid crystal phase in which it selectively reflects light having a desired second wavelength that is shorter than the first wavelength and is then rapidly cooled such that it becomes fixed in the state where it reflects light having the second wavelength. Using this method, gradual cooling from an isotropic phase is not needed, and therefore, the temperature control for image writing is made simpler, enabling fast recording.
For the heat reversible recording medium, a recording medium comprising a recording layer including a cholesteric liquid crystal compound, which is placed on a base, the surface of which has at least a light absorbing capability, may be used.
For the cholesteric liquid crystal compound, a liquid crystal compound is used that has the following features: (i) the selective reflection wavelength range is in the 380 nm to 800 nm range both when the temperature rises and when it falls, and (ii) the longest selective reflection wavelength when the temperature falls is longer than the longest selective reflection wavelength when the temperature rises. Alternatively, a liquid crystal compound may be used which changes to a second wavelength reflective selection state at a temperature that is included in the temperature range in which the compound exhibits a cholesteric liquid crystal phase when the temperature rises.
The recording device pertaining to the present invention has a first recording unit that fixes all or part of the cholesteric liquid crystal compound of the heat reversible recording medium, including a cholesteric liquid crystal compound in a state in which the liquid crystal compound reflects light having a first wavelength. The recording device pertaining to the present invention also has a second recording unit that fixes part of the cholesteric liquid crystal compound of the recording medium, which may be a portion of the part that was fixed in the first wavelength reflection state or may be a different part, in a state in which the liquid crystal compound reflects light having a second wavelength, which is shorter than the first wavelength.
Using the recording device pertaining to the present invention, all or part of the cholesteric liquid crystal compound is fixed in advance by means of the first recording unit in a state in which it reflects light having a desired first wavelength, and therefore, by performing writing so that the liquid crystal compound becomes fixed by the second recording unit in a state in which it reflects light having a desired second wavelength that is shorter than the first wavelength, recording by means of light having the first wavelength as well as light having the second wavelength may be completed, and recording using desired multiple reflection colors may be made.
The first recording unit may include a first heater that heats the cholesteric liquid crystal compound of the recording medium until all or part of it exhibits an isotropic phase, a first cooler that cools the cholesteric liquid crystal compound, which is in an isotropic phase, to the prescribed temperature at which it exhibits a cholesteric liquid crystal phase in which it selectively reflects light having a first wavelength, and a first rapid cooler that rapidly cools the liquid crystal compound, which is at the prescribed temperature, so that it becomes fixed in the state in which it reflects light having the first wavelength. The first heater may comprise heat rollers or may also comprise a thermal head or laser scanning head.
The second recording unit may include a second heater that heats part of the cholesteric liquid crystal compound of the recording medium fixed in the first wavelength light reflection state to a cholesteric liquid crystal phase in which it selectively reflects light having a desired second wavelength that is shorter than the first wavelength, and a second rapid cooler that rapidly cools the heated liquid crystal compound and fixes it in the second wavelength light reflection state. The second heater may comprise a thermal head or laser scanning head or may also comprise heat rollers.
The recording device may also have a reset device that resets the cholesteric liquid crystal compound of the recording medium to an opaque state. The reset device may comprise a system that heats the liquid crystal compound to a temperature that is lower than the temperature range in which the liquid crystal compound exhibits a liquid crystal phase when the temperature rises as well as a system that cools it down to fix it in an opaque state. When this method is used, because heating to and/or gradual cooling from a high temperature at which the liquid crystal compound exhibits an isotropic phase is not necessary, resetting may be completed quickly.